4. Conclusions
PLA/PEG, PLA/NBSK/PEG, and PLA/MDF/PEG composite foams
were developed using foam injection molding process and their
microcellular structure and crystallization behaviors were investigated.
The results showed that the incorporation of NBSK and
MDF fibers led to higher cell density, smaller cell size, and narrower
cell size distribution compared to that of foamed PLA/
PEG samples. This was attributed to the cell nucleating effects
of fibers and the increase in the melt strength of PLA/PEG by
the addition of NBSK and MDF fibers. The crystallization induced
by N2 gas and cellulosic fibers during the foaming process has
also positively affected the final foam morphology by enhancing
the melt strength.
The dissolved N2 and the addition of NBSK and MDF fibers
affected the crystallization kinetics of PLA. From high-pressure
DSC analysis, the pressurized N2 increased the degree of crystallinity
of all PLA samples despite its little effect on the crystallization
temperature and half-time. The presence of cellulosic fibers,
especially NBSK, significantly increased the crystallization temperature
and degree of crystallinity and decreased the crystallization
half-time. In the foam injection molding process, the shear stress
and biaxial stretching significantly enhanced the final degree of
crystallinity of the foamed fiber composites by aligning the PLA
molecules along the fibers.
4. ConclusionsPLA/PEG, PLA/NBSK/PEG, and PLA/MDF/PEG composite foamswere developed using foam injection molding process and theirmicrocellular structure and crystallization behaviors were investigated.The results showed that the incorporation of NBSK andMDF fibers led to higher cell density, smaller cell size, and narrowercell size distribution compared to that of foamed PLA/PEG samples. This was attributed to the cell nucleating effectsof fibers and the increase in the melt strength of PLA/PEG bythe addition of NBSK and MDF fibers. The crystallization inducedby N2 gas and cellulosic fibers during the foaming process hasalso positively affected the final foam morphology by enhancingthe melt strength.The dissolved N2 and the addition of NBSK and MDF fibersaffected the crystallization kinetics of PLA. From high-pressureDSC analysis, the pressurized N2 increased the degree of crystallinityof all PLA samples despite its little effect on the crystallizationtemperature and half-time. The presence of cellulosic fibers,especially NBSK, significantly increased the crystallization temperatureand degree of crystallinity and decreased the crystallizationhalf-time. In the foam injection molding process, the shear stressand biaxial stretching significantly enhanced the final degree ofcrystallinity of the foamed fiber composites by aligning the PLAmolecules along the fibers.
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